A current mirror is a type of current amplifier which provides a high impedance output current proportional to an input current. The output current is typically used to drive a load for high gain. A simple mirror generally consists of a single input and a single output transistor pair, with the gate electrodes of the pair being tied together and to an input voltage node at the drain of the input transistor. The sources of the transistors are connected to a reference voltage node which is common to both. The drain and gate of the input transistor are connected to a current source which provides a quiescent reference current. Because the input and output transistors have their gates and sources tied together, a corresponding output current arises in the conduction path of the output transistor. Generally, the input and output transistors are identical and there is a substantially unity gain in the current. Such mirrors are commony used to provide active loads in high gain amplifier stages.
There are presently two conflicting trends in the design of MOS circuits. One is a trend toward MOS (metal-oxide-silicon) devices with shorter conduction channel lengths for accommodating higher signal frequencies. The other is a trend toward lower supply voltages for reducing power consumption, so that more devices may be included in a single circuit, on a chip. The conflict arises in that as the devices of a current mirror have their channel lengths shortened, their transconductance rises, but their output conductance rises even faster. The resulting lower available current mirror output impedance has led to combined arrangements of two or more mirrors in which the output transistors are connected in series. These, however, require increased power supply voltage, or overhead, for obtaining increased output impedance because each of the output transistors requires sufficient drain-to-source voltage to be biased in saturation.